As technology development and demand for mobile devices increase, demand for secondary batteries as energy sources is sharply increasing. Among the secondary batteries, a lithium secondary battery that has high energy density and voltage, a long cycle lifespan, and a low self-discharge rate is commercialized and being widely used.
However, the lithium secondary battery has a problem in that its life sharply decreases as charge and discharge are repeated. Particularly, such a problem is more serious at high temperature. This is a phenomenon that occurs due to decomposition of an electrolyte, deterioration of an active material, and an increase in an internal resistance of a battery due to moisture in the battery or other reasons.
Accordingly, a positive electrode active material for a lithium secondary battery that is currently being vigorously researched, developed, and used is LiCoO2 having a layered structure. Although LiCoO2 is used the most due to its excellent lifespan characteristics and charge/discharge efficiency, there is a limitation for LiCoO2 to be applied to a technology for increasing battery capacity due to its low structural stability.
As a positive electrode active material for substituting for LiCoO2, various lithium transition metal oxides such as LiNiO2, LiMnO2, LiMn2O4, LiFePO4 and Li(Nix1Coy1Mnz1)O2 have been developed. Among these, LiNiO2 has an advantage of exhibiting a high discharge capacity as a battery characteristic. However, LiNiO2 has problems in that synthesis is difficult with a simple solid state reaction and thermal stability and cycle characteristics are low. Also, lithium-manganese-based oxides such as LiMnO2 and LiMn2O4 have advantages including excellent thermal stability and low cost. However, lithium-manganese-based oxides have problems including low capacity and low high-temperature characteristic. Particularly, LiMn2O4 is commercialized in some low-cost products but has an inferior lifespan characteristic due to structural deformation (Jahn-Teller distortion) caused by Mn3+. Also, a large amount of research is currently being carried out on LiFePO4 for use in hybrid electric vehicles (HEVs) due to low cost and excellent stability. However, it is difficult for LiFePO4 to be applied to other fields due to low conductivity.
Due to such circumstances, a material that is currently being spotlighted the most as a positive electrode active material for substituting for LiCoO2 is a lithium-nickel-manganese-cobalt-based oxide, Li(Nix2Coy2Mnz2)O2 (here, x2, y2, and z2 are atomic fractions of independent oxide-forming elements, and 0<x2≤1, 0<y2≤1, 0<z2≤1, and 0<x2+y2+z2≤1). This material has advantages in that the material is less expensive than LiCoO2 and can be used at high capacity and high voltage. However, the lithium-nickel-manganese-cobalt-based oxide has disadvantages in that a rate capability and lifespan characteristic at high temperature are inferior.
The lithium secondary battery using the above-described positive electrode active material generally has a problem in that safety of a battery is deteriorated or a lifespan characteristic is sharply deteriorated due to an exothermic reaction accompanied by deterioration of a surface structure of the active material and sudden structural collapse as charge and discharge are repeated. Particularly, such a problem is more serious under high temperature and high voltage conditions. This is because an electrolyte is decomposed due to moisture inside a battery or other influences or the active material is deteriorated due to instability of a positive electrode surface, and interface resistance between the electrode including the active material and the electrolyte is increased.
To solve such a problem, methods of improving the structural stability and surface stability of an active material itself by doping or surface-treating the positive electrode active material and improving interfacial stability between an electrolyte and the active material have been proposed. However, the methods are not satisfactory in terms of their effects and processability.
Also, with increasing demand for high capacity batteries nowadays, there is a growing need for development of a positive electrode active material capable of improving battery safety and lifespan characteristic by securing internal structure and surface stability.